Studies on experimental animals and in the clinic indicate that the pyrimidine anti-metabolites PALA (NSC 224131, N-phosphonacetyl-L-aspartate) and 5-fluorouracil have some selectivity for colon tumors and other tumors refractory to many chemotherapeutic agents. Although their mechanisms of action are known, the reasons for their selectivity are not. A comprehensive survey of pyrimidine nucleotide biosynthesis in both host and tumor tissues is proposed to establish these reasons. New methods for determination of amounts of blood-borne pyrimidines (uridine, uracil and cytidine) are being developed, and the relative utilization of these by enzymes of the salvage pathway will be compared with utilization of other metabolites via the de novo pathway for biosynthesis of the uridine and cytidine nucleotides, RNA and DNA. The effects of PALA and uptake of 5-FU (singly and in combination) on these parameters will be determined in tumor and host tissues, using mouse colon-26 and Lewis Lung tumors (which respond to PALA), L1210 mouse ascites tumor (which does not) and rat Novikoff tumor (unknown response). The premise to be tested is that PALA (which almost completely blocks the de novo pathway in both host and tumor) causes a decrease in blood-borne pyrimidine precursors and is most toxic to the tissues most dependent on the alternative salvage pathway; these same tissues would be expected to take up 5-FU the most readily. Knowledge of the determinant parameters for selectivity of PALA and 5-FU should be extendable to other pyrimidine antimetabolites and to treatment of tumors in humans.